Dinoflagellate Alexandrium catenella is a cosmopolitan bloom-forming species with complex life cycle, the formation and germination of resting cysts are critical for its bloom dynamics. In the coastal waters of Qinhuangdao, A. catenella has been identified as the major causative agent for paralytic shellfish poisoning, but there is little knowledge concerning its resting cysts in this region. In this study, three surveys were carried out along the coast of Qinhuangdao from 2020 to 2021 to map the distribution of A. catenella resting cysts, using a quantitative PCR (qPCR) assay specific for A. catenella. The resting cysts were detected in surface sediments during all the three surveys, and their distribution patterns were similar. High abundance of resting cysts (maximum 1 300 cysts/g sediment (wet weight)) were found in a region (119.62°E–119.99°E, 39.67°N–39.98°N) northeast to the coastal waters of Qinhuangdao, where surface sediments were mainly composed of clay and silt (percentage above 50%). Prior to the formation of the A. catenella bloom in March 2021, the abundance of A. catenella vegetative cells in seawater had extremely significant positive correlation with the abundance of resting cysts in surface sediments, reflecting the important role of resting cysts in the initiation of A. catenella blooms. As far as we know, this is the first report on the distribution of A. catenella cysts along the coast of Qinhuangdao. The results will offer a sound basis for the future monitoring and mitigation of toxic A. catenella blooms and paralytic shellfish poisoning events in this region.
The effect of combining vacuum preloading and low-energy dynamic consolidation is predominantly controlled by the tamping interval time. It is, therefore, imperative to identify a suitable parameter and optimise the tamping interval time. In this study, the effect of the tamping interval time on the consolidation of slurries was investigated by conducting laboratory model tests. Consequently, various tamping interval times were obtained by controlling the dissipation of pore water pressure at different levels of 20%, 40%, 60%, 80%, and 100% to determine the optimal interval time. In terms of surface settlement variation, dissipation of pore water pressure, and distributions of the water content and shear strength variation profile, it was shown that a rubber soil state was reached at a dissipation rate of 20% pore water pressure. When the tamping interval time was set to a dissipation rate of 80% pore water pressure, the consolidation effect was optimal.